The present invention relates to industrial borescopes, especially flexible borescopes of the type that can be employed to inspect the condition of a gas turbine or jet engine.
Borescopes of this type have elongated flexible probes, which can have either a miniature video camera at the distal tip or a fiber optic imaging system extending the length of the probe. These probes are employed to penetrate into an inaccessible area of a jet engine or other object to view a target area inside it. Flexible borescopes are often employed to inspect vanes of a stator row in a turbine for foreign object damage. An aeronautical jet engine typically has an inspection port that receives a guide tube which extends from outside the engine to a position near the first stator row. The flexible probe is inserted through the guide tube and then is passed through the vanes of the first stator row into the rotor. There, the tip of the flexible probe latches onto a trailing edge of one of the rotor blades, or else is wedged between two adjacent rotor blades. The jet engine maintenance operator can then observe the leading edges of the vanes in the next stator row as the rotor is slowly turned. This procedure typically permits inspection of one half of the stator vanes in the second row. Then, the rotor is reversed one half turn and the probe is removed. Following this, the probe and guide tube are inserted into a second inspection port and the remaining stator vanes in the second row are inspected in a similar fashion. In some engines, only one inspection port is available so that the borescope must be sufficiently long to inspect all the stator vanes in the second row. Inspection by means of a flexible borescope permits the most susceptible parts of the jet engine or gas turbine to be inspected without disassembling the engine.
A flexible probe which employs an inflatable bladder mechanism for lodging the borescope tip between rotor vanes is described in U.S. Pat. No. 3,841,764. Probes of this type require an inflatable bag or bladder, an inflating mechanism, and an air tube which runs the length of the flexible probe beneath its outer sheath.
Another borescope employs a forceps hook to latch onto the trailing edge of the rotor blades, and is described in U.S. Pat. No. 4,847,817. In this arrangement, an aluminum hook is screwed onto an elongated flexible cable that passes through a tool forceps channel within the insertion tube. In arrangements of this type, the hook can become difficult to manipulate as the operator is required to rotate the cable within the forceps channel to orient the hook. At the same time, the operator must also steer the probe and manipulate its insertion tube.
Both of the above-mentioned prior systems require the borescopes to have additional channels within them, either to inflate the bladder, or to carry the hook cable. Consequently, the borescope insertion tube is of a greater diameter and is somewhat more complex and costly than if the additional channels were omitted.
A hooking cap for borescopes has been described in my copending U.S. Pat. application Ser. No. 451,016 filed Dec. 15, 1989, and having a common assignee herewith. In that case, a hooking cap is removably attached onto the distal tip of the borescope insertion tube; the hooking cap has a flange that can be latched onto and removed from a projection such as the trailing edge of one of the blades of the rotor of a jet engine being inspected and/or serviced. In the various embodiments of the hook cap disclosed in this earlier patent application, the hook cap has a tubular sleeve that overfits the tip of the borescope and a flange that projects radially out from a distal end of the sleeve. In one version the hook flange is annular, but in other versions it can be lobate. The engaging side of the flange can be flat, or can be bent back proximally. As there disclosed, the sleeve can have one or more inwardly projecting lips that engage corresponding transverse slots on the insertion tube tip. The lips are constructed to release from the slots if a predetermined axial breakaway force is exceeded. The hook cap is entirely of aluminum or another material that will be consumed in combustion if it is lost from the borescope tip, and so it will not cause foreign object damage to the jet engine or turbine.
These hook caps are cast aluminum or machined aluminum, and can be difficult to produce to tolerance, except at considerable cost. Also, the finite thickness of the sleeve can push the probe's line of sight away from the engine axis, thus rendering some portions of the stator vanes difficult to see clearly. Moreover, because the hook flange protrudes from both sides of the probe tip, the hook can engage the rotor blade backwards or can wedge into the space between blades. This can increase the frequency of hook cap loss, making consumption of the hook caps much higher than it ought to be.